The present invention relates to a method for testing CRT displays, such as closed circuit television monitors, computer displays and other precision CRT displays, and relates specifically to a method for testing the brightness levels of the raster scan display. In CRT displays of this type it is possible that with time the components of the apparatus will age and the brightness performance of the display will change. When such CRT displays are used in precision equipment, it is necessary to periodically measure the brightness levels of such precision CRT displays to confirm the operation and verify performance to be that typical for the particular type of equipment.
Traditionally, brightness levels of CRT displays have been difficult to measure accurately due to limitations of the instrumentation as well as the methods employed for measuring. The instruments commonly used for measuring brightness levels of a CRT display are luminance meters which are also known as photometers. Such luminance meters are actually spot brightness meters which employ photocells and are extremely precise. Typically, their optical systems are intended to survey a cone of approximately one degree of viewing angle.
The difficulty with measuring the brightness levels of a CRT display with such a spot brightness meter is that the CRT display is formed by the scanning of the screen by an electron beam. As the electron beam scans the screen, the electrons of the beam will impact the spot on the screen and will excite the screen phosphors of that spot and thus, as the spot scans the screen a display having areas of varying brightness is produced. However, the bright spot produced at the point of the incidence of the scanning beam with the screen, has an instantaneous brightness that is very high as compared to the average brightness of the screen. Thus, as the scanning spot passes the area being surveyed by the luminance meter a transient will result in the output reading of the luminance meter which distorts and causes inaccuracies in the brightness measurement. It is therefore desired that the inaccuracies caused by the electron beam scanning spot be eliminated and that the average brightness of the screen be measured rather than the instantaneous brightness caused by spot scanning with an electron beam. Therefore, it is desired that a relatively large area of the screen be surveyed to obtain an average brightness reading.
Another problem in the traditional brightness level testing of a CRT is that the traditional display pattern used for measuring brightness is a bar pattern wherein the screen is divided into ten vertical bars each having a different degree of brightness. This pattern is generally referred to as the Gray Scale pattern. The ten vertical bars each occupy ten percent of the screen width. Since the luminance meter should only view one bar at a time, and since the luminance meter views a circular area, the area surveyed can only be a relatively small circular area having a diameter of one tenth of the screen width. This is too small an area to give accurate readings due to the bright scanning spot passing the surveyed area. As pointed out above, it is desired to survey a substantial portion of the screen so that the average brightness of that portion is measured. If a Gray Scale pattern is used, and a large area is surveyed, the area would contain bars of several different brightness levels and would therefore be incapable of providing meaningful test data.
A further problem in testing brightness levels of a CRT display is that, as the brightness level of the area surveyed by the luminance meter is varied from high to low, the current drawn from the high voltage supply by the electron beam will change as different current levels are necessary to generate different brightness levels. Since normally the high voltage supply of a CRT is limited in the amount of power it can supply, this change in current will affect the voltage generated by the supply, which in turn will affect the remaining circuitry of the cathode ray tube and generate a distorted display which cannot be calibrated accurately. As more current is drawn from the high voltage supply by the electron beam in generating a bright display image having a relatively large amount of high illumination image area compared to the amount of background area, the voltage will drop in the high voltage supply. When this voltage drops, it causes a shift in the magnetic field of the deflection control system for the electron beam thereby affecting centering of the image, and it may also cause a difference in the deflection angle, therefore affecting the size of the image. For example, consider an image generated on the screen, such as a technical drawing wherein the lines of the drawing are highly illuminated and occupy a relatively low percentage of the screen and the background is at a low or black illumination level. If the polarity of the image is reversed, wherein the background is at a high illumination level and the lines of the drawing at a low illumination level, there will likely be a shift in the position of the image and also a possible change in its size. This is because when reversing the polarity, much more current will be drawn by the electron beam thereby causing a voltage drop in the high voltage supply. This change in voltage effects the strength of the beam control magnetic field thereby resulting in the displacement of the image on the screen. This phenomenon can result in inaccuracies in the image displayed which can be quite significant depending upon the application. It is therefore desired that the current drawn from the high voltage supply be relatively constant so that no distortion will occur in the display. Therefore, it is desired that the average picture level or APL of the CRT display be maintained a constant proportion of the peak picture level. By holding the APL constant the current drawn from the voltage supply will remain constant, the measurements taken will be accurate, and proper calibration can result.